It is often necessary to erect scaffolding for repair, construction, inspection, cleaning, or other maintenance within large cavernous structures having very small access holes thereto. This type of situation is very commonly encountered in the power, paper, and other industries where fossil fueled steam boilers are commonly utilized to produce steam for generating electrical power, for chemical processing, or the like. It is often necessary to shut down such boilers for cleaning and general maintenance, as well as for emergency repairs. Since these structures involve large capital investment and result in substantial financial loss when not in operation, it is important that the time for servicing such boilers be minimized. Lost income can be as high as several hundred thousand dollars per day of shut down. Merely saving one day in scaffolding erection time can more than offset the total cost of the scaffolding. Consequently, anything which can increase the speed of erecting temporary scaffolding within such structures by more efficiently handling material as is necessary within such structures has significant commercial value.
This has been a recognized need in this industry for at least ten years, and probably considerably longer.
A limiting factor in such boilers or similar equipment is the very small size of the access openings to the interior. For example, the inner walls of modern boiler furnaces contain an array of steam generating tubes protectively lining all of the walls. The interruption of this array is minimized and is typically limited to a few manholes which may typically range in size from 18" to 30" in diameter (or along a side, if a square or rectangular opening).
Anything which is to be used in the interior of such boilers must pass through these access holes, and where appropriate should also be readily removable from the interior after the temporary use is completed, so that such equipment will not be subjected to the destructive high temperatures typical in the interior of the boiler during normal operation.
In earlier years, temporary custom-made wooden scaffolding would have been used in such boilers. At the present time, this would be prohibitively expensive because of the high cost and limited life of wood and because of the high labor costs due to labor-intensive requirements and the need for more highly paid semi-skilled workmen.
For a time, suspended scaffolding of the types shown in British Patent Nos. 822,327 (1955) and 1,123,841 (1964) were utilized. However, such suspended scaffolding limited the number of persons who could work in the boiler interior at any one time. This was not desirable for those installations where downtime could result in revenue losses of hundreds of thousands of dollars per day.
Somewhat more recently, by at least the 1970's, tube and coupler type erected scaffolding was adopted for such jobs for reasons of efficiency and safety. This also was time consuming, and required skilled laborers capable of following detailed planning and layout with careful measuring.
This was in turn replaced by modularized post-and-runner type scaffolding which can be erected faster and requires less skill. One method of using such equipment in slope-bottomed boilers was to weld brackets to hold the bottom posts in place on the sloping walls of the bottom of the boiler. See U.S. Pat. No. 4,496,026 showing another modified method of using this modularized post-and-runner equipment in boilers.
While the use of quick erecting modularized post-and-runner scaffolding equipment in such temporary installations has been a boon to the industry (see for example, applicant's assignee's U.S. Pat. No. 4,493,578, which is incorporated herein by reference), there has continued to be a serious drawback in using such equipment in boilers, etc. because of the bottleneck caused by the small access openings. The dismantled scaffolding equipment was not only required to be passed into the interior of the boiler through the small manholes, but was personally carried into place by individual workers or through a "human chain" method of internal material delivery. These cavernous structures can often be thirty feet by fifty feet and rise to several hundred feet and require an array of scaffolding filling the structures with platforms every six feet from the bottom level to the upper portions thereof. Thus, filling the entire structure with scaffolding by hand-to-hand methods, is very slow, very labor intensive, constitutes a safety problem, and therefore, is very costly.
The usual cranes, elevator hoists, and the like typically found at most construction sites are not available within such large structures because of the limitations of the small access openings. Several solutions to these problems have been proposed over the years without any proving feasible. These have included the attempted use of small buckets, canvas slings, endless conveyors, and the like. Yet the inefficient human chain method has persisted over such attempts in spite of its drawbacks.